Addition-amount controller for exhaust gas purifying agent and exhaust emission control system

ABSTRACT

In an addition-amount controller for an exhaust gas purifying agent to be used for an exhaust emission control system of an engine, one mode is executed based on satisfaction of an execution condition for each mode, from among a plurality of control modes. The control modes includes a purification control mode in which an addition amount of NH 3  or an additive serving as a generating source of the NH 3  is determined according to a predetermined parameter associated with an amount of NO x  in the exhaust gas, and a storage control mode in which the addition amount is set to be larger than that in the purification control mode. When a load on an output shaft of the engine is higher than an allowable level, the execution condition of the storage control mode is determined to be satisfied.

CROSS REFERENCE TO RELATED APPLICATION

This application is based on Japanese Patent Application No. 2007-168407filed on Jun. 27, 2007, the contents of which are incorporated herein byreference in its entirety.

FIELD OF THE INVENTION

The present invention relates to an addition-amount controller for anexhaust gas purifying agent, for controlling an amount of addition ofNH₃ for purifying exhaust gas by reaction with NO_(x) in the exhaustgas. The invention also relates to an exhaust emission control systeme.g., a urea-SCR system, for purifying exhaust gas by an exhaust gaspurifying reaction based on NH₃ on a catalyst.

BACKGROUND OF THE INVENTION

In recent years, urea-SCR (selective reduction) systems have beendeveloped in electric power plants, various factories, vehicles, and thelike. Particularly, in the field of vehicles (especially, a dieselengine vehicle), post treatment techniques of exhaust gas for purifyingand reducing NO_(x) (nitrogen oxides) in the exhaust gas are classifiedinto two important trends, namely, the above-mentioned urea-SCR system,and a NO_(x) storage-reduction catalyst. The urea-SCR system is alreadyput into practical use in large trucks, and known to have a highpurification ratio of a maximum of about “90%”. Presently, the generalurea-SCR systems which are now studied for application to diesel enginesare designed to reduce (purify) NO_(x) in the exhaust gas by means ofNH₃ (ammonia) generated from a urea ((NH₂)₂CO) aqueous solution(hereinafter referred to as a urea water).

Conventionally, the system disclosed in JP-A-2003-293739 is known as aspecific example of such a urea-SCR system. This system mainly includesa catalyst for promoting a specific exhaust gas purifying reaction(reduction reaction of NO_(x)), an exhaust pipe for guiding the exhaustgas discharged from an exhaust gas generating source (for example, aninternal combustion engine) to the catalyst, and a urea water additionvalve disposed at a midway point of the exhaust pipe for injecting andadding the urea aqueous solution (additive) to the exhaust gas flowingin the exhaust pipe. The system with this arrangement is configured toinject and add the urea aqueous solution into the exhaust gas by theurea water addition valve, and to supply the urea aqueous solution tothe catalyst on the downstream side together with the exhaust gas, usinga flow of the exhaust gas. The urea aqueous solution thus supplied ishydrolyzed by exhaust gas heat or the like to generate NH₃ (ammonia), asrepresented by the following chemical equation: (NH₂)₂CO+H₂O→2NH₃+CO₂.This leads to a reduction reaction of NO_(x) by the NH₃ on the catalyst,through which the exhaust gas is purified.

However, the catalyst used in such purification of the exhaust gasgenerally promotes the reduction reaction of NO_(x) in a temperaturerange exceeding an activation temperature (critical reactiontemperature) inherent to the catalyst, that is, a temperature rangehaving the activation temperature as the lower limit. Thus, the systemas disclosed in JP-A-2003-293739 cannot have a sufficient capacity ofpurifying the exhaust gas when the catalyst is at a low temperaturebelow the activation temperature.

Most of general catalysts for purification of exhaust gas for use in,for example, a vehicle-mounted internal combustion engine or the likehave the activation temperature of about “180° C.”. In contrast, thetemperature of the exhaust gas emitted from the internal combustionengine during idling is generally about “140 to 150° C.”. The increasein temperature of the exhaust gas together with acceleration of theengine heats the catalyst, but the catalyst does not become a hightemperature immediately. In other words, energy transfer is performed toincrease the engine rotation speed, the exhaust gas temperature, and thecatalyst temperature; however the increase of the engine rotation speed,the increase of the exhaust gas temperature, and the increase of thecatalyst temperature are delayed little by little in that order. Thecatalyst also remains at a low temperature for a while after the startof acceleration of the engine. Thus, when the internal combustion engineserving as the exhaust gas generating source starts to accelerate fromthe idling state, the sufficient exhaust gas purification capacity isnot obtained even though the increase in amount of emission of NO_(x) ispredicted due to a high load operation. This may lead to deteriorationof exhaust emission characteristics. The same kind of problem may alsobe posed at other times, including startup of the engine.

SUMMARY OF THE INVENTION

The present invention has been made in view of the forgoing facts, andit is an object of the invention to provide an addition-amountcontroller for an exhaust gas purifying agent, which can obtain a highexhaust gas purification capacity in response to more conditions, and anexhaust emission control system which can exhibit the high exhaust gaspurification capacity by using the addition-amount controller.

According to the present invention, an addition-amount controller for anexhaust gas purifying agent is configured to be applied to an exhaustemission control system for purifying exhaust gas emitted from aninternal combustion engine. The exhaust emission control system includesa catalyst for promoting a specific exhaust gas purification reaction ina temperature range having a critical reaction temperature as a lowerlimit, and an addition valve for adding an additive of NH₃ (ammonia) oran additive serving as a generating source of the NH₃ to the catalystitself or the exhaust gas on an upstream side with respect to thecatalyst, the additive being adapted to purify NO_(x) (nitrogen oxides)in the exhaust gas by the exhaust gas purification reaction on thecatalyst. The addition-amount controller is adapted to control an amountof addition by the addition valve, and the catalyst has properties ofstoring NH₃ and further decreasing the critical reaction temperature asthe amount of NH₃ storage is increased.

According to a first aspect of the present invention, theaddition-amount controller includes: mode selection means for selectingone mode to be executed at that time based on satisfaction of anexecution condition for each mode, from among a plurality of controlmodes, the control modes including a purification control mode in whichthe addition amount by the addition valve is determined according to apredetermined parameter associated with an amount of NO_(x) in theexhaust gas, and a storage control mode in which the addition amount bythe addition valve is set to be larger than that in the purificationcontrol mode; and execution condition determining means for determiningwhether the execution condition of the storage control mode issatisfied. Furthermore, the execution condition determining meansdetermines that the execution condition of the storage control mode issatisfied when a load on an output shaft of the internal combustionengine is higher than an allowable level.

Alternatively, according to a second aspect of the present invention,the addition-amount controller includes: mode selection means forselecting one mode to be executed at that time based on satisfaction ofan execution condition for each mode, from among a plurality of controlmodes, the control modes including a purification control mode in whichthe addition amount by the addition valve is determined according to apredetermined parameter associated with an amount of NO_(x) in theexhaust gas, and a storage control mode in which the addition amount bythe addition valve is set to be larger than that in the purificationcontrol mode; and execution condition determining means for determiningwhether the execution condition of the storage control mode issatisfied. Furthermore, the execution condition determining meansdetermines that the execution condition of the storage control mode issatisfied when a rotation speed of an output shaft of the internalcombustion engine is accelerated from a lower state than an allowablelevel, or from a deceleration state.

Alternatively, according to a third aspect of the present invention, theaddition-amount controller includes: mode selection means for selectingone mode to be executed at that time based on satisfaction of anexecution condition for each mode, from among a plurality of controlmodes, the control modes including a purification control mode in whichthe addition amount by the addition valve is determined according to apredetermined parameter associated with an amount of NO_(x) in theexhaust gas, and a storage control mode in which the addition amount bythe addition valve is set to be larger than that in the purificationcontrol mode; and execution condition determining means for determiningwhether the execution condition of the storage control mode issatisfied. Furthermore, the execution condition determining meansdetermines that the execution condition of the storage control mode issatisfied when an amount of fluctuation in rotation speed of the outputshaft of the internal combustion engine is larger than an allowablelevel.

Alternatively, according to a fourth aspect of the present invention,the addition-amount controller includes: mode selection means forselecting one mode to be executed at that time based on satisfaction ofan execution condition for each mode, from among a plurality of controlmodes, the control modes including a purification control mode in whichthe addition amount by the addition valve is determined according to apredetermined parameter associated with an amount of NO_(x) in theexhaust gas, and a storage control mode in which the addition amount bythe addition valve is set to be larger than that in the purificationcontrol mode; and execution condition determining means for determiningwhether the execution condition of the storage control mode issatisfied. Furthermore, the execution condition determining meansdetermines that the execution condition of the storage control mode issatisfied when an amount of fluctuation in load on the output shaft ofthe internal combustion engine is larger than an allowable level.

Such a controller according to any one of the first to fourth aspects ofthe present invention can enhance the purification capacity of thecatalyst at a low temperature by decreasing the critical reactiontemperature of the catalyst through execution of the above-mentionedstorage control mode. Thus, the critical reaction temperature of thecatalyst can be decreased through the execution of the storage controlmode so as to enhance the purification capacity of the catalyst at a lowtemperature. Furthermore, the mode selection means starts the abovestorage control mode at timing when deterioration of the emissioncharacteristics may be caused. That is, the timing includes timing whena load applied to the output shaft of the internal combustion enginebecomes higher than the allowable level. The timing also includes timingwhen the rotation speed of the output shaft of the internal combustionengine is accelerated from a state in which the rotation speed is lowerthan the allowable level or from a deceleration state, and timing whenthe amount of fluctuation in rotation speed of the output shaft of theinternal combustion engine becomes larger than the allowable level. Thetiming further includes timing when the amount of fluctuation in loadapplied to the output shaft of the internal combustion engine becomeslarger than the allowable level. This can suitably suppress thedeterioration of the emission characteristics due to the exertion of theunnecessary storage of the NH₃.

In general, the temperature of the catalyst has an influence on theproperty of the catalyst (for example, a limit NH₃ storage amount, orthe like). Further, it is known that the higher the temperature of thecatalyst, the less the limit storage amount of NH₃ (limit NH₃ storageamount). Thus, when the catalyst temperature is sufficiently low, anallowance degree up to the limit NH₃ storage amount is large, which mayrequire more storage of NH₃. As mentioned above, when the catalyst is ata low temperature, the activation temperature (critical reactiontemperature) of the catalyst is strongly required to be decreased. Inview this point, the addition-amount controller may be further providedwith catalyst temperature determination means for determining whether ornot the temperature of the catalyst at that time is lower than apredetermined temperature. In this case, the execution conditiondetermining means determines that the execution condition of the storagecontrol mode becomes dissatisfied when the temperature of the catalystdetected by the catalyst temperature determination means is not lowerthan the predetermined temperature during the execution of the storagecontrol mode. Accordingly, it is possible to store the NH₃ in a limitedway in the more demanding condition, that is, when the catalysttemperature is lower than the predetermined temperature.

For example, the addition-amount controller may be further provided withstorage amount determination means for determining whether an amount ofNH₃ storage of the catalyst is larger than a predetermined amount. Inthis case, the execution condition determining means determines that theexecution condition of the storage control mode becomes dissatisfiedwhen the amount of NH₃ storage of the catalyst determined by the storageamount determination means is larger than the predetermined amountduring the execution of the storage control mode.

Alternatively, the addition-amount controller may be further providedwith limit storage determination means for determining whether NH₃ isable to be stored in the catalyst. In this case, the execution conditiondetermining means determines that the execution condition of the storagecontrol mode becomes dissatisfied when the limit storage determinationmeans determines that NH₃ is unable to be stored during the execution ofthe storage control mode.

Alternatively, the execution condition of the purification control modemay be satisfied when the execution condition of the storage controlmode is dissatisfied. In this case, the mode selection means is adaptedto switch between two types of control modes of the purification controlmode and the storage control mode according to satisfaction ordissatisfaction of the execution condition.

According to a fifth aspect of the present invention, an addition-amountcontroller for an exhaust gas purifying agent is configured to beapplied to an exhaust emission control system for purifying exhaust gasemitted from an internal combustion engine. The exhaust emission controlsystem includes a catalyst for promoting a specific exhaust gaspurification reaction in a temperature range having a critical reactiontemperature as a lower limit, and an addition valve for adding anadditive of NH₃ (ammonia) or an additive serving as a generating sourceof the NH₃ to the catalyst itself or the exhaust gas on an upstream sidewith respect to the catalyst, the additive being adapted to purifyNO_(x) (nitrogen oxides) in the exhaust gas by the exhaust gaspurification reaction on the catalyst. The addition-amount controller isadapted to control an amount of addition by the addition valve, and thecatalyst has properties of storing NH₃ and further decreasing thecritical reaction temperature as the amount of NH₃ storage is increased.Furthermore, the addition-amount controller includes: operating modedetermination means for determining whether an operating mode of theinternal combustion engine is a specific operating mode in which a loadon an output shaft of the internal combustion engine is controlled to beincreased when the catalyst is at a low temperature below the criticalreaction temperature; and setting means for setting the amount ofaddition by the addition valve for the NH₃ storage to the catalyst whenthe operating mode is determined to be the specific operating mode bythe operating mode determination means.

The emission characteristics may become deteriorated especially inoperating modes, including an engine startup operation, an accelerationoperation from an idling state (a return to the idling), and further areacceleration operation in which a long-term deceleration operation ona downslope is changed to a slope ascending operation (a return to thefuel cut). That is, the above-mentioned operating mode is an operatingmode in which a load applied to the output shaft of the internalcombustion engine is controlled to be increased when the catalysttemperature is low. According to the fifth embodiment of the presentinvention, the deterioration of the emission characteristics can besuitably restricted, while a decrease in NO_(x) purification ratio dueto the exertion of the unnecessary storage of the NH₃ can be made small.

For example, the addition valve may be adapted to inject and add a ureaaqueous solution as the additive to the exhaust gas on an upstream sidewith respect to the catalyst. In this case, the urea aqueous solution isinjected and added to the exhaust gas on the upstream side with respectto the catalyst, so that the urea is hydrolyzed by exhaust gas heat orthe like until the urea reaches the catalyst to form NH₃. This cansupply more NH₃ (purifying agent) to the catalyst.

BRIEF DESCRIPTION OF THE DRAWINGS

Additional objects and advantages of the present invention will be morereadily apparent from the following detailed description of preferredembodiments when taken together with the accompanying drawings. Inwhich:

FIG. 1 is a schematic diagram showing an addition-amount controller foran exhaust gas purifying agent, and an exhaust emission control systemwith the addition-amount controller, according to one embodiment of theinvention;

FIG. 2 is a flowchart showing control processing for controlling anamount of addition of urea water;

FIG. 3 is a flowchart showing control processing for determining starttiming of an engine acceleration time period;

FIG. 4 is a flowchart showing control processing for determining endtiming of the engine acceleration time period;

FIG. 5 is a graph showing an example of a map used for calculation of alimit NH₃ storage amount;

FIG. 6 is a graph showing an example of a relationship between thecritical reaction temperature of a SCR catalyst and the NH₃ storageamount;

FIG. 7 is a graph showing an example of a purifying property of the SCRcatalyst;

FIGS. 8A to 8C are timing charts showing one form of urea water additioncontrol according to the embodiment; and

FIG. 9 is a flowchart showing another example of control processingregarding mode selection.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

An addition-amount controller for an exhaust gas purifying agent and anexhaust emission control system according to one embodiment of theinvention will be described below with reference to the accompanyingdrawings. The exhaust emission control system of this embodiment has thebasic structure used in a general urea-SCR (selective reduction) system,as an example. With the structure shown in FIG. 1, NH₃ (ammonia)generated from a urea ((NH₂)₂CO) aqueous solution (hereinafter referredto as a urea water) reduces (purifies) NO_(x) in exhaust gas.

Referring to FIG. 1, the structure of the exhaust emission controlsystem will be described in detail below. FIG. 1 is a diagramschematically showing the structure of a urea-SCR system (exhaust gaspurification device) according to this embodiment.

As shown in FIG. 1, this system is adapted to purify exhaust gas emittedfrom a diesel engine (exhaust gas generating source) mounted on, forexample, a four-wheeled vehicle (not shown). The system mainly includesvarious actuators and sensors for purifying the exhaust gas, and an ECU(electronic control unit) 40. The engine of this embodiment (engine ofinterest) is supposed to be a multi-cylinder engine (for example, inlinefour-cylinder engine) mounted on the four-wheeled vehicle (for example,an automatic car). Each cylinder is provided with an injector having afuel injection valve. Fuel supplied to each cylinder by the injectorburns off in the corresponding cylinder. The engine is the so-calledfour stroke (4×piston stroke) reciprocating diesel engine (internalcombustion engine) which is designed to convert energy generated bycombustion of the fuel into a rotational operation to rotate an outputshaft (crankshaft). In other words, in this engine, the cylinder ofinterest at that time is sequentially determined by a cylinderdetermination sensor (electromagnetic pickup) provided in a cam shaft ofan air intake and exhaust valve. One combustion cycle consisting of fourstrokes, namely, suction, compression, combustion, and exhaust, isperformed in a cycle of “720° C.A” at each of four cylinders #1 to #4.Specifically, for example, the respective combustion cycles for the fourcylinders are sequentially executed at the cylinders #1, #3, #4, and #2in that order by shifting the cycle between one cylinder and the nextcylinder by “180° C.A”.

Specifically, various exhaust gas purification devices are disposed inthe exhaust emission control system to form an exhaust gas purificationsystem. The exhaust gas purification devices include a dieselparticulate filter (DPF) 11, an exhaust gas pipe (exhaust gas passage)12, a SCR catalyst 13, an exhaust gas pipe (exhaust gas passage) 14, anda NH₃ catalyst (for example, oxidation catalyst) 15 disposed from theupstream side of the exhaust gas (on the engine side which is an exhaustgas generating source) in that order. Onto a wall surface of the passageat a midway point of the exhaust gas pipe 12, a urea water additionvalve 16 is disposed such that an injection port 16 a opens toward thedownstream side of the exhaust gas. Therefore, an injection port 16 a isdifficult to be dirty with the exhaust gas. The urea water additionvalve 16 is adapted to add (inject and supply) the urea waterpressure-fed into a urea water tank 17 a to the downstream part withrespect to the DPF 11. In this embodiment, the urea water addition valve16 is a so-called electromagnetic driven injection valve whose drivingis electrically controlled by the ECU 40. The addition valve 16 iscontrolled by the ECU 40 so that the urea water serving as an additiveis injected and supplied by a desired addition amount to the exhaust gasflowing in the exhaust gas pipe 12 between the DPF 11 and the SCRcatalyst 13. Thus, the urea water added (or NH₃ after decomposition) issupplied to the SCR catalyst 13 on the downstream side together with theexhaust gas using the flow of exhaust gas (exhaust gas flow).

That is, in this system, addition of the urea water through the ureawater addition valve 16 generates the NH₃ (purifying agent) based on theurea water as indicated by the following decomposition reaction(formula 1) in the exhaust gas. The following NO_(x) reduction reaction(as indicated by the following formula 2) is performed by use of NH₃ onthe SCR catalyst 13, thereby purifying the exhaust gas (purifyingNO_(x)) to be purified.(NH₂)₂CO+H₂O→2NH₃+CO₂  (Formula 1)NO+NO₂+2NH₃→2N₂+3H₂O  (Formula 2)

The excessive NH₃ (surplus NH₃) not consumed in the above reductionreaction (indicated in the formula 2) and flowing into the downstreamside of the SCR catalyst 13 (exhaust pipe 14) is purified through thereaction (indicated by the formula 3) by the NH₃ catalyst 15, andthereby the amount of NH₃ emitted into the atmosphere is decreased. Thetemperature of the exhaust gas on the downstream side of the SCRcatalyst 13 and the amount of NO_(x) (i.e., NO_(x) emission amount)contained in the exhaust gas can be detected (specifically, can becalculated by the ECU 40 based on outputs from the sensors) by anexhaust gas sensor 14 a (incorporating therein a NO_(x) sensor and atemperature sensor) provided in the exhaust gas pipe 14.4NH₃+3O₂→2N₂+6H₂O  (Formula 3)

Next, each of the above-mentioned exhaust gas purification devicesconstituting the exhaust gas purification system of the exhaust emissioncontrol system according to this embodiment will be described in detailbelow.

First, the DPF 11 is a continuously regenerated filter for particulatematter PM removal, that is, for collecting particulate matter (PM) inthe exhaust gas. For example, the DPF 11 can be continuously used byrepeatedly burning and removing (corresponding to a regenerationprocess) the collected PM in post injection or the like after maininjection for mainly generating torque. The DPF 11 supports aplatinum-based oxidation catalyst not shown (in this example, the DPFand the oxidation catalyst are integrally formed with each other, butmay be formed separately). This can remove HC and CO together withsoluble organic fraction (SOF), which is one of the PM components, andalso oxidize a part of NO_(x) (as the ratio of NO to NO₂ (“NO:NO₂”) iscloser to “1:1”, the purification ratio of NO_(x) becomes higher asindicated by the above reaction formula 2).

The SCR catalyst 13 is formed of catalytic metal, such as vanadium oxide(V₂O₅), supported on, for example, a honeycomb structural catalystcarrier. The SCR catalyst 13 has a catalytic action for promoting thereduction reaction (exhaust gas purification reaction) of NO_(x) thatis, the reaction indicated by the above formula 2.

The structure of the urea water addition valve 16 is based on that of afuel injection valve (injector) commonly used in supply of fuel to anengine for a vehicle (internal combustion engine). The structure of theurea water addition valve 16 is well known, and thus will be brieflydescribed below. That is, for convenience of explanation, illustrationof an inside structure of the addition valve 16 will be omitted. Theurea water addition valve 16 incorporates in a valve body, a needledriving portion formed of an electromagnetic solenoid or the like, and aneedle driven by the needle driving portion and reciprocating (movingvertically) in the valve body (housing). The needle is adapted to openand close a necessary number of injection holes formed in an injectionport 16 a at the tip of the valve body, or a circulation route to theseinjection holes. When the electromagnetic solenoid is energized, theurea water addition valve 16 with this arrangement (each element) movesin the direction of opening the valve by driving the needle by use ofthe electromagnetic solenoid according to an electric signal from theECU 40 (for example, a pulse signal by PWM (Pulse Width Modulation)control), that is, according to an injection command from the ECU 40.Thus, the injection port 16 a at the tip of the valve body is opened,specifically, at least one of the injection holes at the injection port16 a is opened, so that the urea water is added (injected) toward theexhaust gas flowing through the exhaust pipe 12. At this time, theamount of addition of the urea water (injection amount) is determinedbased on an energization time of the electromagnetic solenoid (forexample, corresponding to a pulse width of a pulse signal by the ECU40).

On the other hand, a urea water supply system for pressure-feeding theurea water to the urea water addition valve 16 mainly includes a ureawater tank 17 a, and a pump 17 b. That is, the urea water stored in theurea water tank 17 a is pumped by the pump 17 b disposed in the tank 17a, and then pressure-fed toward the urea water addition valve 16. Thepressure-fed urea water is sequentially supplied to the urea wateraddition valve 16 through a pipe 17 c for supply of the urea water.

At this time, foreign matter contained in the urea water is removed by abarrier filter 17 f provided on the upstream side with respect to theaddition valve 16 before the urea water is supplied to the urea wateraddition valve 16. The pressure of supply of the urea water to theaddition valve 16 is controlled by a urea water pressure regulator 17 d.Specifically, when the supply pressure exceeds a predetermined value, amechanical device using a spring or the like allows the urea water inthe pipe 17 c to return to the urea water tank 17 a. In the presentsystem, the supply pressure of the urea water is controlled to remain atthe predetermined value (set pressure) based on the action of theregulator 17 d. The supply pressure of the urea water is not controlledprecisely to be kept at the set pressure even by the action of such aregulator 17 d. In this system, the supply pressure of the urea watercan be detected by the urea water pressure sensor 17 e (specifically,calculated by the ECU 40 based on the sensor output) provided in apredetermined detection position (for example, on the downstream side ofthe regulator 17 d where a fuel pressure is stabilized through thepressure control by the regulator 17 d).

A section for mainly performing control associated with the exhaust gaspurification as an electronic control unit in such a system is the ECU40 (for example, the ECU for control of the purification of exhaust gasconnected to an ECU for control of the engine via a CAN or the like),that is, the addition-amount controller for an exhaust gas purifyingagent according to this embodiment. The ECU 40 includes a well-knownmicrocomputer (not shown), and operates various types of actuators, suchas the urea water addition valve 16, based on detection signals from thevarious sensors to perform various types of control operationsassociated with the exhaust gas purification in the optimal formaccording to the condition of each time. The microcomputer installed onthe ECU 40 basically includes a CPU (central processing unit) forperforming various computations, a RAM (random access memory) serving asa main memory for temporarily storing therein data in the middle of thecomputation, the result of computation, or the like, and a ROM(read-only memory) serving as a program memory. The microcomputer alsoincludes an EEPROM (electrically erasable and programmable read-onlymemory; electrically erasable programmable nonvolatile memory) servingas a memory for data storage, and a backup RAM (RAM fed by a backuppower source, such as a vehicle-mounted battery). Further, themicrocomputer includes signal processors, including an A/D converter anda clock generation circuit, various computation devices, such as aninput/output port, for inputting and outputting signals with theexternal element, a storage device, a communication device, and a powersupply circuit. The ROM previously stores therein various programs and acontrol map associated with the control of the exhaust gas purification,including a program associated with control of an addition amount of theexhaust gas purifying agent. The memory for storing data (for example,EEPROM) previously stores therein various kinds of control data or thelike, including design data for the engine.

In the above description, the structure of the exhaust emission controlsystem of this embodiment has been described in detail. That is, in thisembodiment with this arrangement, NH₃ serving as the purifying agent isadded to the exhaust gas in the form of urea aqueous solution (ureawater) by the urea aqueous addition valve 16. Thus, the urea water isdecomposed in the exhaust gas to form NH₃, and the NO_(x) reductionreaction (indicated by the formula 2) is performed on the SCR catalyst13 based on the thus-generated NH₃ to purify the exhaust gas (exhaustgas from the engine) to be purified. Furthermore, in this embodiment,the processing shown in FIG. 2 is carried out as the control of anaddition amount of the urea water. This processing can obtain the highexhaust gas purification capacity in response to more conditions. Thecontrol of the addition amount of the urea water will be described withreference to FIGS. 2 to 8.

FIG. 2 is a flowchart showing the addition-amount control of the ureawater. A series of control steps in the processing shown in FIG. 2 isbasically performed repeatedly at intervals of a predeterminedprocessing time while a predetermined condition is satisfied byexecuting the program stored in the ROM by means of the ECU 40, forexample, during the time from the startup of the engine to the stoppingof the engine. Values of various parameters used in the processing shownin FIG. 2 are stored in the storage device, such as the RAM or EEPROMmounted on the ECU 40, as occasion arises, and updated at any time ifnecessary.

As shown in FIG. 2, in the control of the urea water addition amount, atstep S10, it is determined whether or not the engine is beingaccelerated, that is, it is determined whether or not the timing at thattime of step S10 is in an engine acceleration time period.

The engine acceleration time period is set by repeatedly performing aroutine processing other than the processing shown in FIG. 2, that is,the series of steps in the processes shown in FIGS. 3 and 4, stored inthe ROM of the ECU 40, at intervals of the predetermined processingtime.

The processing shown in FIG. 3 is to determine the start timing of theengine acceleration time period.

As shown in FIG. 3, in determination of the start timing, at step S31,it is determined whether or not an amount of fluctuation in theaccelerator operation amount toward the positive side (+side) is largerthan an allowable level, specifically, a predetermined determinationvalue. For example, at step S31, it is determined whether theaccelerator operation amount (i.e., an amount of change per unit oftime) is larger than a determination value. The determination process atstep S31 is repeatedly performed. When the amount of fluctuation in theaccelerator operation amount is determined to be larger than theallowable level at step S31, the accelerator pedal used as anaccelerator operation portion is determined to be pushed by the driver.In the subsequent step S32, the timing when the above determination isperformed is set as the start timing of the engine acceleration timeperiod. The above-mentioned engine acceleration time period correspondsto a time period from when the start timing is set at step S32 to whenthe end timing is set.

On the other hand, the processing shown in FIG. 4 is to determine endtiming of the engine acceleration time period.

As shown in FIG. 4, in determination of the end timing, first, at stepS41, it is determined whether or not the engine is being accelerated,that is, whether or not the start timing of the engine acceleration timeperiod is set in the previous step S32 shown in FIG. 3. Only when it isdetermined that the engine is being accelerated at step S41, theprocesses at step S42 and the following steps are carried out.

That is, while the engine is being accelerated, the procedure willproceed to step S42. At step S42, it is determined whether or not thetemperature of the SCR catalyst 13, for example, the catalysttemperature Tc calculated at steps S11 and S12 shown in FIG. 2, islarger than an allowable level, specifically, whether or not thecatalyst temperature is equal to or more than a predetermineddetermination value Ts (Tc≧Ts). The determination process at step S42 isrepeatedly performed. When the temperature of the SCR catalyst 13 isdetermined to be larger than the allowable level at step S42, the engineis determined to be sufficiently warmed up. In the subsequent step S43,the timing when the above determination of step S42 is performed is setas the end timing of the engine acceleration time period. In this way,the engine acceleration time period described above is determined.

When the time is determined to be in the engine acceleration time periodat step S10, the processes at step S11 and the following steps will beperformed. In contrast, when the time is determined not to be in theengine acceleration time period in the same step S10, the procedure willproceed to step S19 a.

In this embodiment, the addition-amount controller selects one of thepurification control mode and the storage control mode to be carriedout. In the purification control mode, the addition amount of the ureawater by the urea water addition valve 16 is determined according to apredetermined parameter about an NO_(x) amount in the exhaust gas,specifically the rotation speed of the output shaft of the engine(engine rotation speed) and the fuel injection amount. In the storagecontrol mode, the addition amount of the urea water by the urea wateraddition valve 16 is set to be larger than that in the purificationcontrol mode, for example, only by increasing an amount required tocover a shortfall with respect to the target value of the NH₃ storageamount. That is, while one of the control modes is not performed, theother is performed. The selection of the control mode (switching betweenthese control modes) is performed based on the result of determinationby the control processes at steps S10 and S13. More specifically, whenthe necessary condition is determined not to be satisfied at any one ofsteps S10 and S13, the storage of NH₃ is determined to be unnecessary,and thus the purification control mode is performed through the controlprocesses in steps S19 a and S20. On the other hand, when the necessaryconditions are determined to be satisfied at both steps S10 and S13, thestorage control mode is performed through the processes in steps S14 toS20 so as to store the NH₃ on the SCR catalyst 13.

That is, when the time is determined not to be in the engineacceleration time period at step S10, the purification control mode isperformed through the processes in steps S19 a and S20. Specifically, atstep S19 a, an addition amount Q of the urea water is obtained accordingto the engine rotation speed and the fuel injection amount using areference map (or a mathematical formula) for calculation of apredetermined addition amount of the urea water. This reference map hassuitable values (optimal values) of the urea water addition amount Qpreviously determined and written therein by experiments or the likeaccording to (or in an appropriate manner to) respective values of theengine rotation speed and the fuel injection amount. The map is stored,for example, in the ROM or the like in the ECU 40. This can obtain thehigh NO_(x) purification ratio. In the subsequent step S20, the ureawater addition valve 16 is driven (energized only for a time periodaccording to the urea water addition amount Q) based on the urea wateraddition amount Q thus obtained.

On the other hand, when the time is determined to be in the engineacceleration time period at the previous step S10, an exhaust gastemperature Tex is detected in the subsequent step S11. For example, theexhaust gas temperature Tex can be actually measured by the exhaust gassensor 14 a. In next step S12, the temperature of the SCR catalyst 13(catalyst temperature Tc) is calculated based on the detected exhaustgas temperature Tex. The catalyst temperature Tc is calculated using,for example, a predetermined map (or a mathematical formula).

Then, at step S13, it is determined whether or not the catalysttemperature Tc calculated in the previous step S12 is smaller than apredetermined determination value Ts (Tc<Ts). The determination value Tscan be set based on examination result or the like, to be suitable tothe execution condition of the storage control mode.

When the catalyst temperature Tc is determined not to be smaller thanthe determination value Ts at step S13, the procedure will proceed tonext step S19 a, in which the purification control mode described aboveis performed. In contrast, when the catalyst temperature Tc isdetermined to be smaller than the determination value Ts at step S13,the storage control mode is performed through the control processes inthe following steps S14 to S20 so as to store NH₃ in the SCR catalyst13.

Specifically, at step S14, first, a present NH₃ storage amount ST1 whichis the NH₃ storage amount at that time of the SCR catalyst 13 isobtained. At this time, the present NH₃ storage amount ST1 is calculatedby another routine. Specifically, an amount of increase or decrease inNH₃ storage amount ΔNH₃ of the SCR catalyst 13 of each time isdetermined based on a difference between the H₃ amount supplied to theSCR catalyst 13 and the amount of consumption of NH₃ on the SCR catalyst13. And the occasional amounts of increase or decrease of the respectivetimes are subsequently summed to be set as the above-mentioned presentNH₃ storage amount ST1 (ST1(present value)=ΣST1(previous value)+ΔNH₃).The above NH₃ amount supplied to the SCR catalyst 13 is calculated basedon, for example, the addition amount of urea water by the urea wateraddition valve 16. In contrast, the consumption amount of NH₃ on the SCRcatalyst 13 is calculated mainly based on the NO_(x) amount emitted fromthe engine and the purification capacity of the catalyst 13. Among them,the NO_(x) amount emitted from the engine can be calculated based on thepredetermined parameter (for example, the engine rotation speed and thefuel injection amount) associated with the operating condition of theengine. On the other hand, the purification capacity of the SCR catalyst13 (reaction rate of the NH₃) can be calculated, for example, using acontrol model of the SCR catalyst 13. The control model for use can be,for example, one or a combination of the following models: a propertymodel showing a relationship between parameters as to a predeterminedproperty; a transfer function showing a correspondence relationshipbetween respective inputs and outputs regarding a level ratio, afrequency-amplitude ratio, a phase difference, a proportion element, adifferential element, an integral element, and a delay element (=Outputsignal/input signal); and a mathematical model in which a predeterminednatural phenomenon is mathematically described.

Then, at step S15, a limit NH₃ storage amount ST21 is calculated basedon the catalyst temperature Tc calculated in the previous step S12. FIG.5 shows an example of a map used for calculation of the limit NH₃storage amount ST21. This map has suitable values (optimal values)previously written therein by experiments. As shown in FIG. 5, the limitNH₃ storage amount ST21 tends to decrease (a NH₃ storage capacity tendsto decrease) with an increasing of the catalyst temperature Tc.

Then, at step S16, a necessary NH₃ storage amount (required NH₃ storageamount ST22, for example, a fixed value) is obtained so as to obtain adesired temperature as the critical reaction temperature (activationtemperature) of the SCR catalyst 13. The required NH₃ storage amountST22 is determined based on the relationship between the criticalreaction temperature of the SCR catalyst 13 and the NH₃ storage amountas shown in FIG. 6 (one example provided by experiments or the like bythe inventors). As indicated by the solid fine RT in FIG. 6, thecritical reaction temperature of the SCR catalyst 13 tends to decreasewith increasing NH₃ storage amount. In the example indicated by thesolid line RT, the desired temperature is supposed to be a criticalreaction temperature T1 with respect to the critical reactiontemperature T0 when NH₃ is not stored. For example, the criticalreaction temperature T1 is a temperature lower than “140° C.” which isthe catalyst temperature supposed in idling, more specifically, forexample, one temperature in a range of “50 to 120° C.”. At this time,the critical reaction temperature (activation temperature) of the SCRcatalyst 13 is an important parameter for determining the purificationproperty of the SCR catalyst 13. FIG. 7 is a graph showing an example ofthe purification property of the SCR catalyst 13. As shown in FIG. 7,the NOx purification ratio of the SCR catalyst 13 largely changes at theboundary of the critical reaction temperature. That is, on the lowtemperature side with respect to the critical reaction temperature, theNO_(x) purification ratio is set to substantially “0”, and the NH₃storage amount is larger than the NH₃ consumption amount consumed by thepurification reaction with NO_(x). In contrast, on the high temperatureside with respect to the critical reaction temperature, the NO_(x)purification ratio basically becomes larger as increasing catalysttemperature (in particular, drastically changes at a temperature nearthe critical reaction temperature RT).

In the following step S17, by comparing the limit NH₃ storage amountST21 obtained at step S15 with the required NH₃ storage amount ST22obtained at step S16, it is determined whether or not the required NH₃storage amount ST22 is smaller than the limit NH₃ storage amount ST21(ST21>ST22). When the relation of ST21>ST22 is determined to besatisfied at step S17, then at the following step S171, the aboverequired NH₃ storage amount ST22 is set as a target NH₃ storage amountST2. On the other hand, when the relation of ST21>ST22 is determined notto be satisfied at step S17, then at step S172, the above limit NH₃storage amount ST21 is set as the target NH₃ storage amount ST2.

Then, at step S18, a difference between the present NH₃ storage amountST1 and the target NH₃ storage amount ST2 is calculated as a shortfallof the NH₃ storage amount ΔST (an amount of storage that is lacking ascompared to the target NH₃ storage amount ST2) (ΔST=ST2−ST1).

Then, a urea water addition amount Q is obtained using the reference mapfor calculation of the predetermined urea water addition amount (thesame one as that used at step S19 a) and the NH₃ storage amountshortfall ΔST at step S19 b. Specifically, the urea water additionamount Q in the storage control mode is a urea water addition amountincreased so as to cover the NH₃ storage amount shortfall ΔST, ascompared to the urea water addition amount in the purification controlmode. In the following step S20, the urea water addition valve 16 isdriven (energized only for a time corresponding to the urea wateraddition amount Q) based on the urea water addition amount Q thusobtained.

FIGS. 8A to 8C are timing charts showing one form of the urea wateraddition control by taking as an example a time when the vehicleequipped with the addition-amount controller and the exhaust emissioncontrol system is accelerated from the idling state. In FIGS. 8A to 8C,FIG. 8A shows the transition of the engine rotation speed of the engine;FIG. 8B shows the transition of the temperature of the SCR catalyst 13;and FIG. 8C shows the presence or absence of execution (ON=execution,OFF=non-execution) of the purification control mode (indicated by thesolid line L1) and the storage control mode (indicated by the solid lineL2).

As shown in FIGS. 8A to 8C, the vehicle is in the idling state until thetiming t1, and then is accelerated at the timing t1 by the driver'spushing down operation of the accelerator pedal. In this case, theamount of fluctuation in the accelerator operation amount is determinedto be larger than the allowable level at step S31 in FIG. 3. In thesubsequent step S32, the timing when the above determination isperformed is set as the start timing of the engine acceleration timeperiod. The control mode is switched from the purification control modeto the storage control mode through the previous process shown in FIG.2.

The temperature of the SCR catalyst 13 begins to increase at the timingt2 that is slightly delayed from the acceleration (timing t1).Thereafter, the temperature of the SCR catalyst 13 continues toincrease. When the temperature of the SCR catalyst 13 is equal to ormore than the determination value Ts at the timing t3, the end timing ofthe engine acceleration time period is set in step S43 shown in FIG. 4,so that the engine acceleration time period starting from the previoustiming t1 is ended. This again switches the control mode from thestorage control mode to the purification control mode through theprevious process shown in FIG. 2.

Thus, in this embodiment, a series of processes shown in FIG. 2 isrepeatedly carried out, so that the NH₃ storage amount of the SCRcatalyst 13 is increased by a shortfall in the predetermined engineacceleration time period from the timing t1 to the timing t3 shown inFIGS. 8A to 8C. Furthermore, the activation temperature of the catalyst13 (critical reaction temperature) is also controlled to an appropriatetemperature (i.e., the critical reaction temperature T1 in the case of“ST21>ST22”). Thus, the exhaust emission control system can obtain thehigher exhaust gas purification capacity even when the catalyst is atthe low temperature, for example, in the startup of the engine, inacceleration of the vehicle from the idling state, and further inreacceleration of the vehicle from the long-term deceleration operationon the downslope to the slope ascending operation.

As mentioned above, the addition-amount controller for an exhaust gaspurifying agent and the exhaust emission control system according tothis embodiment obtain the following excellent effects and advantages.

(1) The addition-amount controller can be suitably applied to theexhaust emission control system for purifying the exhaust gas emittedfrom the internal combustion engine (engine). In this case, theaddition-amount controller includes the SCR catalyst 13 havingproperties of storing NH₃ and further decreasing the critical reactiontemperature (activation temperature) as the amount of NH₃ storage isincreased (see FIG. 6). The SCR catalyst 13 is adapted to promote aspecific exhaust gas purification reaction in a temperature range havingthe critical reaction temperature as the lower limit. Theaddition-amount controller also includes the urea water addition valve16 for adding the additive (urea water) serving as a NH₃ (ammonia)generating source to the exhaust gas on the upstream side with respectto the SCR catalyst 13. The additive is adapted to purify the exhaustgas by the above exhaust gas purification reaction with NO_(x) (nitrogenoxides) in the exhaust gas on the catalyst 13. Furthermore, theaddition-amount controller is adapted to control the amount of additionof the urea water by the urea water addition valve 16. Such anaddition-amount controller for an exhaust gas purifying agent (ECU 40)includes a control program (mode selection means, corresponding to stepsS10 and S13 in FIG. 2) for selecting one mode to be executed at thattime based on satisfaction of the execution condition for each mode,from among a plurality of control modes, including a purificationcontrol mode and a storage control mode. In the purification controlmode, the addition amount of the urea water by the urea water additionvalve 16 is determined according to a predetermined parameter associatedwith the NO_(x) amount of the exhaust gas. In the storage control mode,the addition amount of the urea water by the urea water addition valve16 is set to be larger than that in the purification control mode. Theaddition-amount controller also includes a control program (conditiondetermining means, corresponding to step S31 in FIG. 3) for determiningwhether or not the execution condition of the storage control mode issatisfied. For example, the execution condition of the storage controlmode is satisfied when an amount of fluctuation in a load applied to theoutput shaft of the engine is larger than an allowable level. Morespecifically, the execution condition of the storage control mode can beset to be satisfied when an amount of fluctuation in the acceleratoroperation amount (required torque) is larger than a predeterminedamount. This can enhance the purification capacity of the catalyst bydecreasing the critical reaction temperature of the catalyst throughexecution of the above-mentioned storage control mode. Accordingly, itis possible to suitably suppress the deterioration of the emissioncharacteristics due to the exertion of the unnecessary storage of theNH₃.

(2) In the storage control mode, at step S20 shown in FIG. 2, the NH₃storage amount of the SCR catalyst 13 is controlled to be the target NH₃storage amount ST2 by compensating for the shortfall of the NH₃ storageamount corresponding to a difference between the target NH₃ storageamount ST2 and the present NH₃ storage amount ST1 (i.e., NH₃ storageamount shortfall ΔST) by the processing at step S19 b. Thus, in thestorage control mode, the shortfall of the NH₃ storage amount (NH₃storage amount shortfall ΔST) is compensated, so that the NH₃ storageamount of the SCR catalyst 13 can be set to the target NH₃ storageamount.

(3) At step S20 shown in FIG. 2, while a predetermined condition(conditions in steps S10 and S13) is satisfied, the control of the NH₃storage amount at steps S14 to S20 described above is repeatedlyperformed. With this arrangement, the NH₃ storage amount of the SCRcatalyst 13 can be continuously controlled to an appropriate amount withhigh accuracy while the predetermined condition at steps S10 and S13 issatisfied. Thus, the activation temperature (i.e., critical reactiontemperature) of the catalyst 13 is controlled to an appropriatetemperature.

(4) The addition-amount controller further includes a control program(catalyst temperature determination means, step S13 shown in FIG. 2) fordetermining whether or not the temperature of the SCR catalyst 13 atthat time (catalyst temperature Tc) is lower than the allowable level.The execution condition of the storage control mode is not satisfiedwhen the temperature of the SCR catalyst 13 is determined to be lowerthe allowable level at step S13. Accordingly, it is possible to storethe NH₃ in a limited way in the more demanding condition, that is, whenthe catalyst temperature is sufficiently low (lower than the allowablelevel).

(5) The execution condition of the purification control mode issatisfied when the execution condition of the storage control mode isnot satisfied. That is, at steps S10 and S13 shown in FIG. 2, two typesof control modes, namely, the purification control mode and the storagecontrol mode are switched according to the satisfaction ordissatisfaction of these execution conditions. This can more easily andaccurately achieve the control of the exhaust gas purification.

(6) At step S14 shown in FIG. 2, the amount of increase or decrease inNH₃ storage amount of the SCR catalyst 13 of each time ΔNH₃ isdetermined based on the difference between the NH₃ amount supplied tothe SCR catalyst 13 and the NH₃ consumption amount on the catalyst 13.Further, the increase or decrease amounts of the respective times aresubsequently summed (ST1 (value at this time)=ΣST1 (previousvalue)+ΔNH₃), thereby detecting the present NH₃ storage amount ST1described above. This arrangement makes it possible to accuratelycalculate the amount of increase or decrease in NH₃ storage amount ofeach time and the present NH₃ amount ST1 by determination that theremaining NH₃ is stored on the SCR catalyst 13 based on the revenue andexpenditure of the NH₃ amount.

(7) At step S14 shown in FIG. 2, the amount of consumption of NH₃ on theSCR catalyst 13 is determined based on a predetermined parameterassociated with the operating condition of the engine (for example, theengine rotation speed and the fuel injection amount). With thisarrangement, the NO_(x) amount emitted from the engine, and further theNH₃ consumption amount on the SCR catalyst 13 can be detected moreeasily and accurately.

(8) The addition-amount controller also includes a control program(limit storage amount detection means, corresponding to step S15 shownin FIG. 2) for detecting the limit storage amount of NH₃ that can bestored in the SCR catalyst 13 at that time (limit NH₃ storage amount ST21). The addition-amount controller further includes a control program(steps S17, S171, and S172 shown in FIG. 2) for setting a variable rangeof the target NH₃ storage amount ST2 by using the limit NH₃ storageamount ST21. The limit NH₃ storage amount ST21 is detected by theprocess at step S15 and is set as the upper limit value (guard value).Thus, it is possible to set the limit NH₃ storage amount ST21 as theupper limit, so as to prevent (or suppress) the supply of the excessNH₃.

(9) At step S15 shown in FIG. 2, the limit NH₃ storage amount ST21 isdetected based on the exhaust gas temperature on the downstream sidewith respect to the catalyst 13, which corresponds to the temperature ofthe SCR catalyst 13. Thus, it can detect (estimate) the temperature ofthe SCR catalyst13 with high accuracy.

(10) A temperature lower than the catalyst temperature of “140° C.”supposed in idling, and an NH₃ storage amount corresponding to thetemperature are set as the critical reaction temperature T1, and furtheras the required NH₃ storage amount ST22 (see FIG. 6 for both),respectively. This can surely purify the exhaust gas even when startingto accelerate from the idling state.

(11) The urea water addition valve 16 is configured to inject and addthe urea aqueous solution as the additive for acting as the NH₃generating source, to the exhaust gas on the upstream side (exhaust pipe12) with respect to the SCR catalyst 13 (that is, to achieve theso-called urea SCR system). Thus, the urea aqueous solution is injectedand added to the exhaust gas on the upstream side with respect to theSCR catalyst 13. Therefore, until the urea water reaches the catalyst13, the urea is hydrolyzed by exhaust gas heat or the like to form NH₃.This can supply more NH₃ (purifying agent) to the SCR catalyst 13

(12) The above urea SCR system is installed on the vehicle equipped withthe diesel engine (four-wheeled vehicle in this embodiment). This canimprove the fuel efficiency and decrease the PM by allowing thegeneration of NO_(x) during the combustion process. This can achieve thecleaner diesel vehicle having the high exhaust gas purificationcapacity.

(13) In contrast, the exhaust emission control system includes the SCRcatalyst 13 and the urea water addition valve 16 together with eachprogram (ECU 40), and a urea water supply device (the urea water tank 17a, the pump 17 b, and the like) for supplying the urea aqueous solutionto the addition valve 16. The exhaust emission control system with thisarrangement achieves the exhaust gas purification system having thehigher exhaust gas purification capacity.

The above-mentioned embodiment may be changed in the following way.

According to the applications of the exhaust emission control system,the control process at step S15, S17, S171, or S172 shown in FIG. 2 maybe omitted. In this case, at step S18, the required NH₃ storage amountST22 can be effectively set to the target NH₃ storage amount ST2 as itis.

In the above-mentioned embodiment, at step S31 shown in FIG. 3, theexecution condition of the storage control mode is determined to besatisfied when the amount of fluctuation in the load applied to theoutput shaft of the engine is larger than the allowable level.Specifically, the execution condition of the storage control mode isdetermined to be satisfied when the amount of fluctuation in theaccelerator operation amount (corresponding to the required torque) islarger than the allowable level, but the invention is not limitedthereto. For example, the same condition may be determined to besatisfied when the rotation speed of the output shaft of the internalcombustion engine is accelerated from a lower level than the allowablelevel, or from the deceleration state. That is, in this case, forexample, at step S31, it is determined whether or not the enginerotation speed is smaller than a predetermined determination value, aswell as whether or not the amount of fluctuation in the acceleratoroperation amount toward the positive side is larger than a predetermineddetermination value. When the engine rotation speed is determined to besmall and the amount of fluctuation in the accelerator operation amountis determined to be large (the accelerator pedal is determined to bepushed) at step S31, the control procedure will proceed to step S32.Alternatively, it may be determined whether or not the fuel cut is beingperformed, as well as whether or not the amount of fluctuation in theaccelerator operation amount toward the positive side is larger than thepredetermined determination value at step S31. When the fuel cut isbeing performed and the amount of fluctuation in the acceleratoroperation amount is determined to be large (the accelerator pedal isdetermined to be pushed) at step S31, the control procedure will proceedto step S32.

Alternatively, the execution condition of the storage control mode maybe determined to be satisfied when an amount of fluctuation in therotation speed of the output shaft of the engine is larger than anallowable level. In this case, for example, at step S31, it isdetermined whether or not the amount of fluctuation in the enginerotation speed toward the positive (+) side (the amount of fluctuationper unit of time) is larger than the predetermined determination value.When the amount of fluctuation in the engine rotation speed isdetermined to be large at step S31, the control procedure will proceedto step S32.

Alternatively, the execution condition of the storage control mode maybe determined to be satisfied when the load (engine load) applied to theoutput shaft of the engine is higher than an allowable level. That is,in this case, for example, at step S31, it is determined whether or notthe engine load is larger than a predetermined determination value. Whenthe engine load is determined to be large an step S31, the controlprocedure will proceed to step S32. Here, the engine load is forexample, a value actually measured by the cylinder inner pressure sensoror an estimated value based on an accelerator operation amount or thelike. This arrangement can obtain an effect based on the effect in theabove paragraph (1).

The addition-amount controller effectively includes a control program(operating mode determination means) for determining whether or not theoperating mode of the engine at that time is a specific operating mode.In the specific operating mode, the load on the output shaft of theengine is controlled to be increased when the temperature of the SCRcatalyst 13 is lower than the critical reaction temperature (activationtemperature). Specifically, the controller is configured to determinewhether or not the operating mode at that time is the specific operatingmode, for example, at step S31 shown in FIG. 3. With this arrangement,for example, when the operating mode of the engine is determined to bethe specific operating mode, the control procedure will proceed to stepS32. Thus, in the specific operating mode, the storage control mode isperformed so as to set the amount of addition of urea water added by theurea water addition valve 16 for storing NH₃ on the SCR catalyst 13, inthe control process at step S19 b shown in FIG. 2. Accordingly, it canobtain an effect based on the effect described in the paragraph (1). Thespecific operating modes effectively set include, for example, theengine startup operation, the acceleration operation from the idlingstate (the return to the idling), and further the reaccelerationoperation in which the long-term deceleration operation on the downslopeis changed to the slope ascending operation (the return to the fuelcut).

Although in the above-mentioned embodiment, two control modes of thepurification control mode and the storage control mode are switched, theinvention is not limited thereto. Adding another control mode to thesecontrol modes enables selection of one to be executed at that time fromamong three or more types of control modes based on satisfaction of theexecution condition of each mode.

For example, mode selection may be performed through the processingexemplified as shown in the flowchart of FIG. 9. In this example, theuse of a value of a urea water addition control flag F (“0 to 2”)selects one of three types of the purification control mode, the storagecontrol mode, and the urea water non-addition mode.

As shown in FIG. 9, in this example, it is determined whether or not apredetermined execution condition associated with the storage controlmode (storage control execution condition) is satisfied at step S101.When the storage control execution condition is determined to besatisfied at step S101, the urea water addition control flag F is set to“2” at the subsequent step S103.

In contrast, when the storage control execution condition is determinednot to be satisfied at step S101, it is determined whether or not apredetermined execution condition associated with the purificationcontrol mode (purification control execution condition) is satisfied inthe subsequent step S102. When the purification control executioncondition is determined to be satisfied at step S102, the urea wateraddition control flag F is set to “1” at the subsequent step S105. Onthe other hand, when the purification control execution condition isdetermined not to be satisfied at step S102, the urea water additioncontrol flag F is set to “0” at the subsequent step S104.

This can select one to be executed at that time among the three or moretypes of control modes.

Although in the above-mentioned embodiment, the NH₃ storage amount isset corresponding to the predetermined critical reaction temperature T1lower than the catalyst temperature of “140° C.” supposed in idling, asthe required NH₃ storage amount ST22 for use in determination of thetarget NH₃ storage amount ST2, the invention is not limited thereto. Forexample, the required NH₃ storage amount ST22 can be effectively set toa boundary value (convergent point) at which the critical reactiontemperature is not decreased even by increasing the NH₃ storage amountof the SCR catalyst 13. This arrangement can suitably prevent (orsuppress) the excess storage of NH₃ not contributing to the criticalreaction temperature.

In the above-mentioned embodiment, the required NH₃ storage amount ST22is set as the fixed value, but the invention is not limited thereto. Therequired NH₃ storage amount ST22 may be variably set according to thecondition of each time. For example, the storage amount may be set todiffer between at the startup time of the engine and the idling time.Alternatively, the required NH₃ storage amount ST22 may be variably setaccording to a target value of the critical reaction temperature or atarget value of the NO_(x) purification ratio on the SCR catalyst 13.

In the above-mentioned embodiment, the catalyst temperature Tc isdetermined based on the exhaust gas temperature. However, thetemperature of the catalyst itself is not determined, and the exhaustgas temperature may be used as a substitute for the catalysttemperature.

In the above embodiment, at step S13 shown in FIG. 2, it is determinedwhether or not the temperature of the SCR catalyst 13 at that time islower than the allowable level (a predetermined threshold), but theinvention is not limited thereto. For example, at step S13, it may bedetermined whether or not the present NH₃ storage amount ST1 is largerthan an allowable level (a predetermined threshold). Alternatively, itmay be determined whether or not there is a sufficient room more than anallowable level between the present NH₃ storage amount ST1 and the limitNH₃ storage amount ST21 by comparison with another predeterminedthreshold. Any one of both determination means described above may beeffectively used. Alternatively, any combination of the above respectivedetermination means, including the catalyst temperature, may beeffectively used. That is, the addition-amount controller includes acontrol program (storage amount determination means) for determiningwhether or not the NH₃ storage amount of the SCR catalyst 13 at thattime is larger than the allowable level. The execution condition of thestorage control mode is not satisfied (becomes dissatisfied) when theNH₃ storage amount of the SCR catalyst 13 is determined to be largerthan the allowable level by the program during the execution of thestorage control mode. Further, the addition-amount controller includes acontrol program (limit storage determination means) for determiningwhether or not NH₃ can be stored on the SCR catalyst 13 at that time.The execution condition of the storage control mode is not satisfied(becomes dissatisfied) when the storing of NH₃ is determined to beimpossible by the program during the execution of the storage controlmode. This enables the storing of NH₃ more suitably.

The NO_(x) amount in the exhaust gas can be determined not only byestimation from the engine operating state, but also, for example, bythe actually measured value (sensor output) by an NO_(x) sensor or thelike. Furthermore, for example, the NO_(x) amount in the exhaust gas canbe estimated based on the state of the exhaust gas (e.g., exhaust gastemperature detected, for example, by the exhaust gas temperature sensoror the like) or components (for example, an oxygen concentrationdetected by an oxygen concentration sensor or the like).

The kind of the exhaust gas generating source to be purified or thesystem structure can be arbitrarily changed according to the usedconditions or the like.

For example, when the exhaust gas from the engine for a vehicle is anobject to be purified, the invention can be applied not only to acompression ignition diesel engine, but also a spark ignition gasolineengine or the like. Since the compression ignition engine, such as thediesel engine, has the low exhaust gas temperature as compared to thatin the spark ignition engine, the invention is effectively applied tothe compression ignition engine, thereby enhancing the purificationcapacity when the catalyst temperature is low. The invention can also beapplied to a rotary engine or the like other than a reciprocatingengine. Furthermore, the invention can also be applied to purificationof exhaust gas from sources other than the vehicle, that is, forexample, purification of exhaust gas from an electric power plant,various factories, or the like.

On the other hand, the system structure may be changed in the followingway. For example, as shown in FIG. 1, the additive (urea water) is addedto the exhaust gas on the upstream side with respect to the catalyst 13to deliver the additive to the catalyst 13 by the exhaust gas flow, butthe invention is not limited thereto. Alternatively, the additive may bedirectly added (for example, injected) to the catalyst itself. Forexample, when the amount of emission of NH₃ is sufficiently decreased inthe structure shown in FIG. 1, the NH₃ catalyst 15 can be omitted fromthe structure.

When various modifications are made to the structures in the aboveembodiments, the details of various processes (programs) described aboveare preferably changed (have designs changed) to the respective optimalforms according to the actual structure if necessary.

Actually, the main demand for the invention comes from the urea-SCR(selective reduction) system. The invention, however, can also be usedfor other applications as long as the exhaust gas is purified on acatalyst using the same purifying agent (NH₃) for purifying the samespecific component of interest.

In the above-mentioned and modified examples, various types of software(programs) are supposed to be used, but hardware, such as a dedicatedcircuit, may be used to achieve the same function.

Such changes and modifications are to be understood as being within thescope of the present invention as defined by the appended claims.

1. An addition-amount controller for an exhaust gas purifying agent, thecontroller being applied to an exhaust emission control system forpurifying exhaust gas emitted from an internal combustion engine, theexhaust emission control system including a catalyst for promoting aspecific exhaust gas purification reaction in a temperature range havinga critical reaction temperature as a lower limit, and an addition valvefor adding an additive of NH₃ (ammonia) or an additive serving as agenerating source of the NH₃ to the catalyst itself or the exhaust gason an upstream side with respect to the catalyst, the additive beingadapted to purify NO_(x) (nitrogen oxides) in the exhaust gas by theexhaust gas purification reaction on the catalyst, the addition-amountcontroller being adapted to control an amount of addition by theaddition valve, the catalyst having properties of storing NH₃ andfurther decreasing the critical reaction temperature as an amount of NH₃storage is increased, the addition-amount controller comprising: a modeselection unit configured to select one mode to be executed at that timebased on satisfaction of an execution condition for each mode, fromamong a plurality of control modes, the control modes including apurification control mode in which the addition amount by the additionvalve is determined according to a predetermined parameter associatedwith an amount of NO_(x) in the exhaust gas, and a storage control modein which the addition amount by the addition valve is set to be largerthan that in the purification control mode; and an execution conditiondetermining unit configured to determine whether the execution conditionof the storage control mode is satisfied, wherein the executioncondition determining unit determines that the execution condition ofthe storage control mode is satisfied when a temperature of the catalystbecomes lower than a predetermined temperature, and when a load on anoutput shalt of the internal combustion engine becomes higher than anallowable load; wherein a required NH₃ storage amount is variably setaccording to a target value of a NO_(x) purification ratio on thecatalyst.
 2. The addition-amount controller for an exhaust gas purifyingagent according to claim 1, further comprising a catalyst temperaturedetermination unit configured to determine whether or not thetemperature of the catalyst at that time is lower than the predeterminedtemperature, wherein the execution condition determining unit determinesthat the execution condition of the storage control mode becomesdissatisfied when the temperature of the catalyst detected by thecatalyst temperature determination unit is not lower than thepredetermined temperature during the execution of the storage controlmode.
 3. The addition-amount controller for an exhaust gas purifyingagent according to claim 1, further comprising a storage amountdetermination unit configured to determine whether an amount of NH₃storage of the catalyst is larger than a predetermined amount, whereinthe execution condition determining unit determines that the executioncondition of the storage control mode becomes dissatisfied when theamount of NH₃ storage of the catalyst determined by the storage amountdetermination unit is larger than the predetermined amount during theexecution of the storage control mode.
 4. The addition-amount controllerfor an exhaust gas purifying agent according to claim 1, furthercomprising a limit storage determination unit configured to determinewhether NH₃ is able to be stored in the catalyst, wherein the executioncondition determining unit determines that the execution condition ofthe storage control mode becomes dissatisfied when the limit storagedetermination unit determines that NH₃ is unable to be stored during theexecution of the storage control mode.
 5. The addition-amount controllerfor an exhaust gas purifying agent according to claim 1, wherein theexecution condition of the purification control mode is satisfied whenthe execution condition of the storage control mode is dissatisfied, andwherein the mode selection unit is adapted to switch between two typesof control modes of the purification control mode and the storagecontrol mode according to satisfaction or dissatisfaction of theexecution condition.
 6. The addition-amount controller for an exhaustgas purifying agent according to claim 1, wherein the addition valve isadapted to inject and add a urea aqueous solution as the additive to theexhaust gas on an upstream side with respect to the catalyst.
 7. Anexhaust emission control system comprising: the addition-amountcontroller as in claim 6; the catalyst and the addition valve; and aurea water supply device for supplying the urea aqueous solution to theaddition valve.
 8. The addition-amount controller for an exhaust gaspurifying agent according to claim 1, further comprising: an operatingmode determination unit configured to determine whether an operatingmode of the internal combustion engine is a specific operating mode inwhich the load on the output shaft of the internal combustion engine iscontrolled to be increased when the catalyst is at a low temperaturebelow the critical reaction temperature; and a setting unit configuredto set the amount of addition by the addition valve for the NH₃ storageto the catalyst when the operating mode is determined to be the specificoperating mode by the operating mode determination unit.
 9. Theaddition-amount controller for an exhaust gas purifying agent accordingto claim 1, wherein the mode selection unit selects the storage controlmode when the execution condition determining unit determines that theexecution condition of the storage control mode is satisfied, and themode selection unit selects the purification control mode when theexecution condition determining unit determines that the executioncondition of the storage control mode is not satisfied.
 10. Theaddition-amount controller for an exhaust gas purifying agent accordingto claim 1, wherein a required NH₃ storage amount is variably setaccording to the critical reaction temperature of the catalyst.
 11. Theaddition-amount controller for an exhaust gas purifying agent accordingto claim 1, wherein the NH₃ storage amount is set to differ between at astartup time of the engine and an idling time.
 12. An addition-amountcontroller for an exhaust gas purifying agent, the controller beingapplied to an exhaust emission control system for purifying exhaust gasemitted from an internal combustion engine, the exhaust emission controlsystem including a catalyst for promoting a specific exhaust gaspurification reaction in a temperature range having a critical reactiontemperature as a lower limit, and an addition valve for adding anadditive of NH₃ (ammonia) or an additive serving as a generating sourceof the NH₃ to the catalyst itself or the exhaust gas on an upstream sidewith respect to the catalyst, the additive being adapted to purifyNO_(x) (nitrogen oxides) in the exhaust gas by the exhaust gaspurification reaction on the catalyst, the addition-amount controllerbeing adapted to control an amount of addition by the addition valve,the catalyst having properties of storing NH₃ and further decreasing thecritical reaction temperature as an amount of NH₃ storage is increased,the addition-amount controller comprising: a mode selection unitconfigured to select one mode to be executed at that time based onsatisfaction of an execution condition for each mode, from among aplurality of control modes, the control modes including a purificationcontrol mode in which the addition amount by the addition valve isdetermined according to a predetermined parameter associated with anamount of NO_(x) in the exhaust gas, and a storage control mode in whichthe addition amount by the addition valve is set to be larger than thatin the purification control mode; and an execution condition determiningunit configured to determine whether the execution condition of thestorage control mode is satisfied, wherein the execution conditiondetermining unit determines that the execution condition of the storagecontrol mode is satisfied when a temperature of the catalyst becomeslower than a predetermined temperature, and when a rotation speed of anoutput shaft of the internal combustion engine is accelerated from alower speed lower than an allowable speed or from a deceleration state;wherein a required NH₃ storage amount is variably set according to atarget value of a NO_(x) purification ratio on the catalyst.
 13. Theaddition-amount controller for an exhaust gas purifying agent accordingto claim 12, further comprising: an operating mode determination unitconfigured to determine whether an operating mode of the internalcombustion engine is a specific operating mode in which a load on anoutput shaft of the internal combustion engine is controlled to beincreased when the catalyst is at a low temperature below the criticalreaction temperature; and a setting unit configured to set the amount ofaddition by the addition valve for the NH₃ storage to the catalyst whenthe operating mode is determined to be the specific operating mode bythe operating mode determination unit.
 14. The addition-amountcontroller for an exhaust gas purifying agent according to claim 12,wherein the mode selection unit selects the storage control mode whenthe execution condition determining unit determines that the executioncondition of the storage control mode is satisfied, and the modeselection unit selects the purification control mode when the executioncondition determining unit determines that the execution condition ofthe storage control mode is not satisfied.
 15. The addition-amountcontroller for an exhaust gas purifying agent according to claim 12,wherein the required NH₃ storage amount is variably set according to thecritical reaction temperature of the catalyst.
 16. The addition-amountcontroller for an exhaust gas purifying agent according to claim 12,wherein the NH₃ storage amount is set to differ between at a startuptime of the engine and an idling time.
 17. An addition-amount controllerfor an exhaust gas purifying agent, the controller being applied to anexhaust emission control system for purifying exhaust gas emitted froman internal combustion engine, the exhaust emission control systemincluding a catalyst for promoting a specific exhaust gas purificationreaction in a temperature range having a critical reaction temperatureas a lower limit, and an addition valve for adding an additive of NH₃(ammonia) or an additive serving as a generating source of the NH₃ tothe catalyst itself or the exhaust gas on an upstream side with respectto the catalyst, the additive being adapted to purify NO_(x) (nitrogenoxides) in the exhaust gas by the exhaust gas purification reaction onthe catalyst, the addition-amount controller being adapted to control anamount of addition by the addition valve, the catalyst having propertiesof storing NH₃ and further decreasing the critical reaction temperatureas an amount of NH₃ storage is increased, the addition-amount controllercomprising: mode selection means for selecting one mode to be executedat that time based on satisfaction of an execution condition for eachmode, from among a plurality of control modes, the control modesincluding a purification control mode in which the addition amount bythe addition valve is determined according to a predetermined parameterassociated with an amount of NO_(x) in the exhaust gas, and a storagecontrol mode in which the addition amount by the addition valve is setto be larger than that in the purification control mode; and executioncondition determining means for determining whether the executioncondition of the storage control mode is satisfied, wherein theexecution condition determining means determines that the executioncondition of the storage control mode is satisfied when a temperature ofthe catalyst becomes lower than a predetermined temperature and when anamount of fluctuation in rotation speed of the output shaft of theinternal combustion engine becomes larger than an allowable level;wherein a required NH₃ storage amount is variably set according to atarget value of a NO_(x) purification ratio on the catalyst.
 18. Theaddition-amount controller for an exhaust gas purifying agent accordingto claim 17, wherein the required NH₃ storage amount is variably setaccording to the critical reaction temperature of the catalyst.
 19. Theaddition-amount controller for an exhaust gas purifying agent accordingto claim 17, wherein the NH₃ storage amount is set to differ between ata startup time of the engine and an idling time.
 20. An addition-amountcontroller for an exhaust gas purifying agent, the controller beingapplied to an exhaust emission control system for purifying exhaust gasemitted from an internal combustion engine, the exhaust emission controlsystem including a catalyst for promoting a specific exhaust gaspurification reaction in a temperature range having a critical reactiontemperature as a lower limit, and an addition valve for adding anadditive of NH₃ (ammonia) or an additive serving as a generating sourceof the NH₃ to the catalyst itself or the exhaust gas on an upstream sidewith respect to the catalyst, the additive being adapted to purifyNO_(x) (nitrogen oxides) in the exhaust gas by the exhaust gaspurification reaction on the catalyst, the addition-amount controllerbeing adapted to control an amount of addition by the addition valve,the catalyst having properties of storing NH₃ and further decreasing thecritical reaction temperature as an amount of NH₃ storage is increased,the addition-amount controller comprising: mode selection means forselecting one mode to be executed at that time based on satisfaction ofan execution condition for each mode, from among a plurality of controlmodes, the control modes including a purification control mode in whichthe addition amount by the addition valve is determined according to apredetermined parameter associated with an amount of NO_(x) in theexhaust gas, and a storage control mode in which the addition amount bythe addition valve is set to be larger than that in the purificationcontrol mode; and execution condition determining means for determiningwhether the execution condition of the storage control mode issatisfied, wherein the execution condition determining means determinesthat the execution condition of the storage control mode is satisfiedwhen a temperature of the catalyst becomes lower than a predeterminedtemperature and when an amount of fluctuation in load on the outputshaft of the internal combustion engine becomes larger than an allowablelevel; wherein a required NH₃ storage amount is variably set accordingto a target value of a NO_(x) purification ratio on the catalyst. 21.The addition-amount controller for an exhaust gas purifying agentaccording to claim 20, wherein the required NH₃ storage amount isvariably set according to the critical reaction temperature of thecatalyst.
 22. The addition-amount controller for an exhaust gaspurifying agent according to claim 20, wherein the NH₃ storage amount isset to differ between at a startup time of the engine and an idlingtime.